Short stroke electro-magnetic actuator



Dec. 24, 1957 1 E. LAWRENCE 2,817,729

SHORT STROKE ELECTRO-MAGNETIC ACTUATOR Filed Sept. 15, 1954 s sheets-sheet 1 aff-@al INVENTOR 97 50 ATTORNEY Dec. 24, 1957 L. E. LAWRENCE 2,817,729

SHORT STROKE ELECTRO-MAGNETIC AcTuAToR Filed Sept. 13, 1954 3 Sheets-Sheet 2 y 28 32T" 3747 asfvf +7 #152- 34 INVENTOR 4.3 w W l 43 MMM/M76 ATTORNEY 3 Sheets-Sheet 3 ATTORNEY Dec. 24, 1957 L. E. LAWRENCE SHORT STROKE ELECTRO-MAGNETIC ACTUATOR Filed Sept. 15, 1954 United States Patent O SHORT STROKE ELECTRO-MAGNETIC ACTUATOR Leland E. Lawrence, Wauwatosa, Wis., assignor to Allen- Bradley Company, Milwaukee, Wis., a corporation ot Wisconsin Application September 13, 1954, Serial No. 455,695

21 Claims. (Cl. 200-104) This vinvention relates to electro-magnetic actuators suitable for actuating electric switches, valves and for similar purposes and resides more specifically in an actuator which is specially useful for short stroke operation, particularly under alternating current excitation, the actuator including a magnetic frame closed upon itself divided in two places to form a yoke and armature relatively movable to form a pair of working gaps magnetically in series, the gaps each being surrounded by a separate exciting winding concentrated in the vicinity of the `gap to direct and control the position and density of the fiux resulting from excitation of both windings in its respective working gap in a manner which contributes to the creation of increased pull force.

In the employment of an electro-magnetic actuator for switching purposes the work load comprises the weight of the armature and movable contact structure, or other biasforce urging these elements ,to an open position, together with the reaction of the usual contact backing springs that are compressed after initial contact closure. The increase in pull force between the magnetic members, that occurs upon movement of the armature to closed position must be sufficient to meet and overcome the additional load imposed by the contact springs at an intermediate point in the stroke. If the pull force does not rise to meet and overcome this increase in load, improper switch action may occur, with either erosion, overheating or welding of the contacts. lt is desirable therefor in electromagnetic actuators for switching that there be a rapid rise `in pull force during the closing stroke of the armature. To the extent that such can be accomplished the relatively lower pull force in the fully retracted armature position raises the threshold value for the pick-up voltage, which is desirable, and the increasing pull` force aids in obtaining full stroke operation against the increased load of the contact springs whenever excitation is sufficient to cause pick-up to occur.

Accepted practice dictates that a magnetically actuated switch close satisfactorily at voltages applied to the magnet coil substantially less than rated voltage, for example, 85% of rated voltage. The actuator force should match the load at the start of the stroke and again at the intermediate point in the stroke Where the contacts engage, at any acceptable energizing voltage. If, however, the actuator force greatly exceeds the load at either point in the stroke at minimum accepted Voltage, the excess energy and `resulting excess hammering of the sealing surfaces at full voltage will materially shorten the life of the actuator. Since the stroke and contact force are set by the requirements for the switch, the actuator should `have a stroke vs. `force characteristic which approximates the switch requirements and which can be easily adjusted to conform closely with switch requirements. Actuators constructed .in accordance with this invention have a force vs. stroke .characteristic which satisfies these requirements.

In accordance with lthe present invention a .pair of gaps 2,817,729 Patented Dec. 24, 1957 ICC between the magnetic members is provided and windings connected with aiding polarities are concentrated near and placed to closely surround the gaps. The total air gap area is twice the cross section of the magnetic path and for a given flux density the pull force is twice that of a conventional solenoid or other magnet having one working gap.

The ilux density in a magnetic actuator is normally limited or xed by the losses in the sealed position where the leakage fiux is very small. In open stroke position, however, the fiux density in the air gap is reduced in three ways. (l) The coil resistance produces a voltage drop which subtracts from the line voltage thus reducing the voltage available for magnetization. (2) With the air gaps present in the magnetic circuit, magnetic poles are produced and some flux will leak from one pole to the other without crossing the working air gap. Also a certain amount of flux linkage with coil turns will be produced by fiux loops not entering the iron path. Both leakages reduce the total effective fiux in the working air gap and diminish the available pull force. k(3) The effective area of the air gap becomes larger than the pole area because of fringing fiux with a consequent reduction of liux density. The density is reduced by the same factor as the area is increased so a net reduction in pull force results because pull force varies with the square of the density but only directly with the area.

-in the actuator of this invention the above mentioned factors tending to reduce effective air gap ux and air gap fiux density are substantially reduced thus bringing about a marked increase in the effectiveness of the magnet.

lt is an object of this invention to provide an electromagnetic actuator in which the magnetic circuit contains two working air gaps having substantially equal pull force.

lt is another object of this invention to provide an electro-magnetic actuator in which a higher proportion of the total flux available traverses the working gaps of the magnetic circuit.

It is another object of this invention to provide an electro-magnetic actuator in'which the pull force rises rapidly with stroke, upon movement of the magnetic elements, to meet and overcome substantial increases in load encountered during the stroke.

It is another object of this invention to provide an electro-magnetic actuator in which the energy consumed by the operating windings when the magnetic members are sealed is materially reduced as compared with other magnets of equivalent pull force.

lt is another object of this invention to provide an electro-magnetic actuator in which the working air gaps of the magnetic circuit are encircled by magnetizing windings to cause a high proportion of the flux to be usefully disposed within and in relation to the space constituting the working gaps.

It is another object of this invention to provide an electro-magnetic actuator in which a demagnetizing air gap interrupting the magnetic circuit may be disposed where it will retain fixed dimensions without being subject to diminution through wear of the pole faces of the magnetic elements.

lt is another object of this invention to provide an electro-magnetic actuator having a yoke free to move to permit alignment of the pole faces of the magnetic members, upon sealing of the armature with the yoke.

It is another object of this invention to provide an electro-magnetic actuator having a yoke free to move with the armature upon closure thus reducing the force of the impact and absorbing and dissipating the energy of the impact Without damage.

It is another object of this invention to provide a magnet which employs a pair of energizing coils that may 3 be connected to be energized by either of two voltage sources.

It is another object of this invention to provide a simplied mounting for the energizing windings of an electro-magnetic actuator which will, as well, retain the yoke with freedom of movement for alignment of pole faces and absorption of impact and which further acts to guide the armature in its movement.

The foregoing and other objects and advantages of this invention will appear from the description to follow. In the description reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration and not of limitation a specic form in which this invention may be embodied.

In the drawings:

Fig. l is a front view in elevation with parts broken away and in section of a switch including one form of an electro-magnetic actuator in which this invention may be embodied,

Fig. 2 is a side view in elevation of the switch shown in Fig. 1,

Fig. 3 is a side View in elevation and in section of the switch viewed through the plane 3-3 shown in Fig. 1,

Fig. 4 is a top plan view partly in section of the switch viewed through the plane 4-4 shown in Fig. l,

Fig. 5 is a view in perspective of the magnetic elements forming a part of the electro-magnetic actuator for the switch, with the energizing windings of the actuator shown schematically, and

Fig. 6 is a view in perspective of a pair of coil bobbins that form a part of the electro-magnetic actuator, separated and with the coils to be wound upon the bobbins outlined in broken lines.

Referring now to the drawings, there is shown a vertical back plate 1 with a set of mounting holes 2 along the margins suitable for securing the apparatus upon a panel board, or within a housing. Fastened across the upper front of the back plate 1 is a forwardly extending contact hood 3 of molded insulating material. To preserve alignment of the contact hood 3 with respect to the plate 1 a pair of rectangular bosses 4, one of which 1s shown in Fig. 3, extends from the back face of the hood 3 to ilt snugly within complementary openings in the plate 1. A single obliquely disposed mounting bolt 5 passes through the hood 3 to threaded engagement with a tapped hole in the plate 1. By bringing the bolt 5 up tightly the hood 3 is rigidly secured.

The Contact hood 3, in the form shown, includes a back wall 6 extending across the width of the back plate 1. A pair of side walls 7 project forwardly from the ends of the back wall 6 and a forwardly projecting cover wall 8 extends between the top margins of the side walls 7. Extending between the forward ends of the side walls 7 and closing with the front margin of the cover wall o 1s a front wall 9 that completes the exterior of the contact hood 3. The hood 3 thus is constructed in the form of an enclosure for a plurality of Contact sets, the enclosures being closed at the top and on all four sides, but open at the bottom.

The interior of the contact hood 3 is divided into four separate contact compartments by a set of three partitions 10 extending downwardly from the cover wall 8 between the back wall 6 and the front wall 9. The partitions 1lb are evenly spaced one from the other and from the side walls 7, causing the contact compartments to be uniform in dimension.

In each of the compartments, a front terminal 11 is xed to the lower side of the front wall 9 by a mounting bolt 12, as is shown in Fig. 3. Each terminal 11 includes both a downwardly facing fixed contact 13 within the hood 3, and a suitable terminal screw 1d facing the front of the apparatus, so that the screw 14 is accessible from the exterior of the hood 3. A rear terminal 15 is disposed within the hood 3 in each compartment and held in place by a rivet 16. A portion of, each terminal 15 extends upwardly through an opening in die cover wall 8 to threadedly receive a terminal screw 17 at the upper rear exterior of the contact hood 3. Each rear terminal 15 also mounts a downwardly facing fixed contact 18 within the hood 3. The contacts 13 are disposed rto the rear of and in alignment with respective front fixed contact 13, as shown in Fig. 3.

Extending forwardly from the lower portion of the back plate 1 is a rear coil bobbin 19 and extending forwardly from it a front coil bobbin 20. The bobbins 19 and 2d are disposed directly beneath the contact hood 3 in medial relation thereto. The rear of bobbin 19 is seated tlush against the back plate 1 and a pair of mounting bolts 21 passing from the front face of the front bobbin 20 through both bobbins and into the back plate 1, se- Cures the bobbins in the position shown.

As appears in Fig. 6, the rear coil bobbin 19 includes a thin walled sleeve 22 that deiines a vertically extending central opening 23 generally rectangular in cross section. Formed integrally with the sleeve 22 is a relatively massive rectangular upper tlange 24 with side, back and front rails open at the center to form a continuation of the opening 2.3. The llange 24 is recessed at the front to form a step or shoulder 2S at a level beneath that of the main upper surface area of the ilange 24.

The rear coil bobbin 19 also includes a lower rectangular ilange 26, in plan, similar to that of the upper llange 24 but symmetrically inverted so that the front rail 27 of the lower flange 26 is recessed in the same manner as the upper llange 24, but to a slightly greater extent laterally. A set of four abutments 2S extend forwardly from the front corners of the ilanges 24 and 26, and a pair of openings 29 extend through the right side of the upper flange 24 and the left side of the lower flange 26 to re ceive the mounting bolts 21. Tenons 30 project from the left rear of the upper flange 24 and the right rear of lower flange 26 for engagement with complementary mortises in the back plate 1. Similar tenons itl extend from the bobbin 20 to be received in mortises 31 formed in the upper left and lower right abutments 28.

A magnet coil 32 comprising a suitable number of turns of wire shown in phantom in Fig. 5, is wound about the sleeve 22 between the tlanges 24 and 26. The coil 32 is of limited axial extent and is compactly wound and insulated, so as to concentrate the same in a manner and for a pur pose -to be described hereinafter.

The front coil bobbin 20 includes a thin walled sleeve 33 surrounding a vertical central opening 34 of substantially rectangular cross section, like sleeve 22. The sleeve 33 joins with an upper flange 35 and a lower llange 36, similar to the ilanges 24 and 26 of the rear bobbin 19. The upper flange 35 is recessed at the rear rail to form a step or shoulder 37 having an upper surface substantially co-planar with the upper surface of the shoulder 25 of the rear bobbin 19.

The rear rail 38 of the lower flange 36 is of reduced thickness, in the manner of the front rail 27 of the lower flange 26 of the bobbin 19. A set of four abutments 39 extend rearwardly from the rear corners of the flanges 3S, 36 to bear against the abutments 2S of the rear bobbin 19. Protruding rearwardly from the upper left and lower right abutment feet are the tenons 4t) aligned for insertion within the mortises 31 in the abutment feet Z of the rear bobbin 19, as previously mentioned. A pair of openings d1 extend through the flanges 35 and 36 in alignment with the openings 29 in the rear bobbin 19 to pass the mounting bolts 21.

Wound about the sleeve 33 is a second magnet coil 42 similar to coil 32 and electrically connected thereto. Attached to lead ends of the coil 42 are terminals 43 located to the front of the apparatus for suitable connection to a source of energizing voltage.

ln the manufacture of the coil assembly, the coils 32 and 42 are wound and taped before the bobbins 19, 20 are assembled. After assembly the coils may be enclosed by a tape 44, then dipped, or otherwise impregnatedwith asuitable insulating varnish, and baked. The resulting coil assembly upon mounting on the back plate 1 by bolts 21 furnishes not only a sturdy unitary housing for the coils 32 and 42 but also a support and guide for magnetic members to be described.

A yoke 45 of laminated magnetic steel is seated upon and extends between the shoulders and 37 of the upper bobbin flanges 24 and 35. The ends of the yoke 45 overhang the shoulders 25 and 37 and are turned downwardly to extend within the central sleeve openings 23 and 34. The downwardly facing ends of the yoke 45 terminate in magnet poles 46 having precisely ground sealing faces disposed within the central openings of each of the coils 32 and 42. The yoke 45 rests freely upon the shoulders 25, 37 and is retained against major lateral displacement by the inner surface areas of the bobbin flanges 24, 35, clearance being provided, however, between the yoke 45 and flanges 24, 35 so that substantial tilting of the yoke in either a sidewise or fore and aft direction is permitted. If desired, the upper surface of the shoulders 25, 37 may be slightly crowned to facili-tate such tilting. It

is to be particularly noted, however, that yoke 45 is free to move bodily upwardly for purposes to be described.

Disposed below the bobbins 19, 20 is an armature 47 of laminated magnetic steel shaped to present a pair of upturned ends extending within Ithe central sleeve openings 23 and 34 and terminating in a pair of magnetic poles 48 with upwardly facing sealing surfaces adapted to seal against the poles 46 of the yoke 45. The armature 47 is downwardly biased by gravity toward a lower position, shown in Fig. 2, wherein the magnetic laminations forming the armature 47 are beneath the bobbins 19 and 20, with the exception of the upwardly turned poles 48. The armature 47 may be raised, upon sufficient excitation of the magnet coils 32, 42 to the position, shown in Fig. 3, where the armature 47 moves into the recess provided by the reduced front rail 27 of the bobbin 19 and the reduced rear rail 38 of the bobbin 20.

Armature 47 may comprise twin stacks of L-shaped laminations separated from one another to form a transverse demagnetizing air gap 49 of fixed dimension that lies outside of the central openings of the coils 32 and 42. In certain instances the demagnetizing gap 49 may be dispensed with. The armature laminations are tightly held between end plates 50 of nonmagnetic stainless steel from the medial portion of which vertically extending actuator bars 51 emerge. The actuator bars 51 rise from the sides of the armature 47 and are spaced to pass on either side of the yoke 45. The bars 51 furthermore are guided for substantially vertical travel by the closely adjacent abutments 2S and 39 on the outer sides of the bars 5'1 that limit sidewise movement, and by the iianges of the bobbins 19, 20 adjacent the edges of the arms 51 that limit forward and backward movement. Thus, the bobbin assembly guides the armature as well as supporting both of the magnetic elements and the magnet coils. The upper ends of the bar 51 extend above the yoke 45 and are notched to form hooks 52 suitable for engagement with a contact cross bar 53. The sealing surfaces of the poles 48 are recessed, as shown in Figs. 3, 4 and 5, to receive shading rings 54.

As appears more clearly in Fig. 3, the hooks 52 of the attachment bars 51 engage a complementary ridge 55 beneath the Contact cross bar 53 and are held in place by a clamp plate 56 brought up tightly against the back edges of the bars 51 by a pair of mounting bolts 57 that extend through the cross bar 53 into threaded engagement with tapped openings in the plate 56.

The Contact cross bar 53 spreads from the attachment bars 51 to underlie the compartments of the contact hood 3, formed by the partitions 10. A set of four arches 58, one of which is more clearly shown in Fig. 1, extend upwardly from the cross bar 5,3, and inserted `through each arch is a contact bridge 59 having .a V,pair of upwardly facing contacts ,6,0 in position to be engaged with and disengaged from its corresponding set of stationary contacts 13 and 18, A prestressed spring 61 seated beneath` each contact bridge 59 urges the same upwardly against a stop 62 that for-ms an integral portion of the respective arch 65). Initial closure of the contacts 60 with the stationary contacts 13, 18 occurs before the armature 47 seals with the yoke 45, and continued upward movement of the armature 47 causes compression of the springs 61 to provide requisite contact pressure, which pressure is maintained upon sealing. Thus, the load placed upon the electro-magnetic actuator in mid-stroke of the armature 47 suddenly increases by the amount of the reaction of springs 61 as soon as the contact bridges depart from the stops 62.

The upward stroke of the armature 47, caused by excitation of the coils 32, 42, nally carries the armature poles 48 against the poles 46 of the yoke 45 with impact. The yoke 45 being free to rise is thereby momentarily lifted from the bobbin shoulders 25 and 37, and the absence of any direct attachment between the yoke 45 and the bobbin assembly allows the yoke 45 to yield to the impact and to shift as may be required to align the sealing faces at its poles 46 with the sealing faces of the armature 47. At the same time, some of the energy of impact is ab sorbed by friction attendant upon movement of yoke 45 and peak impact stresses are minimized.

The armature 47 also is guided for vertical movement with some lateral freedom which supplements the foregoing means for accommodating misalignment of the movable and stationary parts. Thus, the yoke 45 may accommodate whatever attitude the armature 47 may assume, and upon sealing of magnetic members 45, 47, the yoke 45 returns to rest upon the crowned face of the bobbin shoulders 25, 37 for support of the magnetic members and for maintenance of the switch closing force.

Upon deenergization of the coils 32, 42 the armature drops until the underside of the contact cross bar 53 strikes the yoke 45 where it is arrested in the position shown in Fig. 1. The stroke of the armature 47 is thus limited, and as indicated by Fig. 2 the working air gaps between the yoke and armature poles are wholly conned within the central openings of the coils 32, 42 for all armature positions.

As stated, the axial extent of the coils 32 and 42 is iimited. The purpose of so proportioning the coils is to concentrate the magneto-motive force thereof in the vicinity of the working air gaps. This concentration of the magneto-motive force acts to urge fringing flux back under the pole faces 46, 48 thus raising ux density. A turn of the coil contributes to this effect to a substantial extent so long as it is not spaced axially from the nearest pole face more than about twice the length of the open working air gap. Turns of the coils 32 and 42 placed beyond this, influence the field configuration at and in the vicinity of the air gap but little, even though such turns contribute to induction of the flux.

The advantages of this invention therefor are obtained by placing substantially all or at least a major part of the turns of coils 32 and 42 so as to perform two functions, namely, (l) to induce flux and (2) to control and direct the flux which it and the other coil produces, in a manner which reduces fringing, increases flux density in the working air gaps and increases pull force.

,theprojected area of an adjacent pole, such area being measured on a plane transverse to the mean direction of ux at the center of the working gap. The term short stroke as used herein refers to a stroke approximating this relationship. Within the limits given, desirable results have been achieved, in which substantial economy not only of materials has been made possible but the energy expended in maintaining the magnet in closed position had been reduced in relation to magnets previously known.

ln an actuator constructed in accordance with this invention, the iron section for a given pull force, at short strokes, corresponding to contact engagement in a switch, may be less than half that required in other designs. The closely encircling coils employed with the magnet of this invention may, therefore, have a shorter mean turn length and correspondingly less voltage drop for a given wire size, or if the same voltage drop may be tolerated, advantage may be taken in the form of savings through use of a coil having less copper.

In the actuator of this invention the leakage paths are relatively long and of small cross section. This results in relatively low leakage flux and a larger' portion of the total liux is effectively directed through the working gaps. Computations based on measured pull characteristics show that the leakage flux may be as low as approximately of that found in electro-magnets heretofore employed. Since both working air gaps of the magnet of this invention are closely surrounded by the concentrated windings, the ux is largely confined to the air gap and the increase in effective pole area due to fringing flux is held to a minimum. As stated this results in maximum density in the air gaps and maximum pull force.

Illustrative ofthe advantage to be gained from actuators constructed in accordance with this invention, comparison may be made between an instance of the magnet of this invention and a magnet heretofore regarded as commercially acceptable, but somewhat larger in physical size. With this magnet as heretofore constructed as a reference basis the magnet constructed in accordance with this invention exhibits approximately the following advantages:

Iron weight about 56% less Copper weight about 65% less Pull force at PAG" stroke 100% more Seal watts input about 42% less, and

Pull force per lb. wt. of iron and copper about 378% more In preferred constructions employing this invention, the coil length exceeds the stroke length but little, if any, in which case, the magnetomotive force even more favorably directs, focuses or contines the flux within and across the working gap keeping density high. For small actuators having very short strokes, adequate coils of such short length may be difficult to wind but the advantages of the invention may be obtained nevertheless to a substantial degree even though somewhat longer coils are employed.

Placing the center of the air gap at the center of the coil results in minimum leakage flux and maximum pull force. It is, therefore, desirable to place the center of air gap existing at the point of contact engagement at the center of the coil.

ln some applications it may be desirable to provide a pull force at full open stroke which is quite small, thus ensuring a relatively high pick-up voltage for the armature. This characteristic may be introduced by permitting the armature poles to come to rest in open position a short distance outside the coil openings. As engagement is approached the working gaps will, however, be surrounded by the coils, in a manner availing of the advantages of this invention.

The invention is particularly adapted for use with alternating current excitation. To provide sufficient reluctance in the magnetic path to ensure proper drop out of the armature upon deenergization of such an alternating current magnet the lixed air gap 49, previously described, may be disposed transversely of the magnetic laminations at a position remote from the coil openings. Because of this location the flux is not directed and confined through and within the gap 49, as in the working gaps, thus permitting the use of a relatively larger and less critical gap at this point. Because the mass of iron in relation to pull force is small, total residual flux is much reduced in a magnet of this invention and demagnetzation may be suffi ient to allow opening of the magnet, where the opening bias is substantial, without provision of a demagnetizing gap. This is of particular advantage where very simple, compact and rugged magnets are required for operation under alternating current excitation.

By concentrathg the coils in a position surrounding the working gaps a further advantage is obtained by the magnet of this invention since a novel relationship in shading is brought about. The suppression of fringing produced by windings so placed, offsets, to a substantial degree, 'undesirable field distortions which are inevitably introduced, otherwise, by the presence of the shading windings 54. The magnet of this invention, with two shaded poles, each within a coil proportional as described, may therefore be constructel to provide increased pull force, when fully shaded, as compared with a magnet not possessed of this advantage.

I claim:

l. in an electro-magnetic actuator the combination cornprising a magnetic frame providing a tlux path closed on itself divided at two places 'to form a yoke and a relatively movable armature scalable with said yoke and separable therefrom to present a pair of working gaps that are cach restricted to a length within one and one-half times the square root of the area of an adjacent pole, and a coil with a central opening for each of said working gaps to be connected to a source of alternating current of substantially constant voltage, said coils being disposed with the coil openings encircling the working gaps and being concentrated about the gaps to conne the flux resulting from excitation thereof to thereby increase the ux density in the working gaps, said magnetic frame being interrupted by a fixed gap in addition to said working gaps that is outside of the coil openings.

2. In an electro-magnetic actuator the combination comprising a magnetic yoke having a lengthwise medial portion and a pair of legs extending in like direction from opposite ends of the medial portion terminating in a pair of pole faces, a magnetic armature supported for limited relative movement having a lengthwise medial portion in substantial parallelism to the medial portion of said yoke with a pair of legs extending from opposite ends of the medial portion of said yoke with a pair of legs extending from opposite ends of the medial portion toward the legs of said yoke terminating in a pair of pole surfaces adpated to be sealed with and moved away from the pole faces of said yoke upon relative movement of the armature to form a pair of working gaps between the yoke and armature, the relative movement of the armature away from the yoke being no greater than one and one-half times the square root of a pole face area, and a pair of coils for connection to a source of alternating current of substantially constant voltage each encircling one ot said working gaps, there being an interruption in the flux path provided by said yoke and armature in addition to said working gaps that is outside said coils.

3. ln an electromagnetic actuator the combination comprising a yoke support with upwardly facing surface areas, a magnetic yoke resting upon said surface areas and having a pair of ends turned and facing downwardly whereby said yoke is free to move upwardly upon being struck upon said ends by an upwardly directed blow, an

armature beneath said yoke having a pair of upwardly 9 Iillr'rizd ends spaced from the .ends ofsaid yoke for contacting engagement therewith upon an upward stroke of the armature which armature ends are normally spaced from the ends of said yoke to define a pair of working gaps therebetween that are each of a length within one and one-half times the square root of the projected area of an adjacent pole on a plane transverse to the direction of armature travel, and a pair of coils one for each of said working gaps disposed in closely surrounding concentrated relation to said gaps to direct and confine the flux of both through said gaps to increase liux density, excitation of said coils raising said armature to move toward and strike said yoke whereby the impact momentarily raises said yoke from said upwardly facing surface areas to permit said yoke to align the ends thereof with the armature ends in a sealed position.

4. An electro-magnetic actuator in accordance with claim 3 wherein the yoke and armature provide a flux path that is interrupted by a fixed gap in addition to the working gaps which fixed gap is outside said coils.

5. An electro-magnetic actuator in accordance with claim 3 wherein the upwardly facing yoke supporting surface is convex to facilitate aligning movement of said yoke.

. v6. VIn an electro-magnetic actuator the combination comprising a yoke support member presenting a bearing surface, a magnetic yoke urged toward and normally resting against said bearing surface displaceable therefrom upon being .struck Vby an impact blow and presenting a pair of poles, a magnetic armature having a pair of poles in facing relation to said poles of said yoke movable into and away from engagement therewith, means limiting the stroke of the armature to within the square root of a pole face area, and tv/o windings each having a length no greater than twice the square root of a pole facearea and adapted to encircle a pole of said armature and a pole of .said yoke whereby energization of said windings moves said armature into engagement with said yoke with an impact that carries the yoke with the armature and momentarily unseats the yoke from the bearing surface of said yoke support member to allow the yoke to'shift position for alignment of the yoke pole faces with the poles of ysaid armature.

7. In an electro-magnetic actuator the combination comprising a first coil bobbin having a sleeve with a central opening and flanges at the ends of the sleeve, and a first coil wound about the sleeve and between the flanges of said first bobbin; a second coil bobbin having a sleeve with a central opening and flanges at the ends of the sleeve, and a second coil wound about the sleeve and lbetween the flanges of said second bobbin, said bobbins being secured to one another with the central openings thereof vertically disposed; a magnetic yoke extending from bobbin to bobbin with downwardly turned pole ends inserted in said bobbin openings free to be displaced upwardly and to be tilted about axes normal to one another; and an armature beneath said bobbins with upwardly turned pole ends movable into said bobbin openings upwardly to strike said yoke ends upon energization of said coils to displace said yoke for alignment of the pole ends of the yoke with the pole ends of the armature, the stroke of the armature from its lowermost position to the point of contact with said yoke being restricted to a distance within the square root of a pole face area.

8. In an electro-magnetic actuator the combination comprising a coil form having a pair of bobbins with central openings spaced in substantial parallelism, windings encircling each of said bobbins, an armature at one end of the bobbin openings having pole faces entering the openings to be surrounded by said windings and having an actuator to the outside of and adjacent the windings extending through the coil form in a line substantially parallel to the bobbin openings that is in bearing engagement with the coil form for guided armature travel, and a yoke supported by the coil form at the end of the bobbin openings opposite the armature having pole faces turned into the bobbin openings for engagement with the armature pole faces and to be surrounded by said windings, the working gap between the yoke pole faces and the armature pole faces when the armature is at rest being less than one and one-half times the square root of a pole face area.

9. In an electro-magnetic switch the combination comprising a magnetic frame providing a fiux path closed upon itself divided at two places to form a yoke and an armature relatively movable in a stroke toward and from said yoke to present a pair of working gaps, means limiting armature movement to restrict the working gaps to within one and one-half times the square root of the area of an adjacent pole face, a set of stationary contacts, a set of movable contacts carried by said armature making initial engagement with said stationary contacts in an intermediate position in the stroke of the armature, a set of contact bias springs cooperating with said movable contacts interposing a load on said magnetic frame upon engagement of the movable and stationary contacts, and a pair of coils for connection to a source of alternating current of substantially constant voltage, one coil surrounding each gap with the mid-pointof the length of each coil substantially co-incident with the mid-point of the length of the gap when the armature is in the stroke position of initial contact engagement, each coil being Aconcentrated about its respective gap to control and direct a substantial part of the flux resulting from excitation of the coils so as to increase fiux density in said working gaps.

10. In an electro-magnetic actuator a magnetic frame closed upon itself to form a fiux path loop that is divided into two relatively movable segments with a pair of poles for each scalable with the opposite poles of the other, means guiding said relative movement to provide two substantially like working gaps disposed between the poles, stop means limiting the length of each working gap to a distance less than one and one-half times the square root of the projected area of an adjacent pole on a plane transverse to the mean direction of fiux passing through the working gap, and a pair of coils one for each working gap concentrated and disposed to surround its respective gap to control and direct a substantial part of the flux resulting from excitation of both of said coils so as to increase the flux density in said working gaps.

1l. An electro-magnetic actuator in accordance with claim l() wherein the coils are concentrated so that the turns thereof most remote axially from the poles are spaced axially therefrom not more than the square root of the projected area of the pole.

12. In an electromagnetic actuator the combination comprising a pair of coils with a pair of openings disposed substantially parallel to one another, a magnetic yoke extending between coil openings having a pair of ends turned into said openings to present pole faces disposed therein, a magnetic armature extending between coil openings at the ends thereof opposite said yoke and having a pair of ends turned into said openings to present pole surfaces movable toward and away from engagement with said yoke pole faces, means limiting the movement of the armature to a stroke not more than the square root of a pole face area and a pair of actuator arms joined to said armature on opposite sides thereof between its pole surfaces extending outside of and adjacent the coils to a position beyond said yoke.

13. An apparatus in accordance with claim l2 having guide means in bearing engagement with each of said actuator arms for confining the same to substantial translatory movement so as to preserve substantially equal working gaps between said `yoke and armature in all positions of said armature when away from sealing posi tion.

14. In an electromagnetic actuator the combination comprising a pair of coils having substantially parallel central openings and windings adapted to be connected to an alternating current source having a predetermined rated value, a magnetic yoke extending between coil openings with a pair of ends turned into said openings to pres` ent pole faces disposed therein, a magnetic armature extending between coil openings at the ends of the coils opposite said yoke and having a pair of ends turned toward the coil openings to provide pole surfaces movable into and away from engagement with said yoke pole faces, means limiting the movement of the armature to a stroke not more than one and one-half times the square root of a pole face area, a pair of actuator arms joined to said armature having straight, parallel guiding surfaces maintained transverse to said pole surfaces during armature movement, and guiding ways for the armature in bearing engagement with the guiding surfaces of said actuator arms adapted to guide the armature in substantially straight line motion to maintain substantially equal working gaps between yoke and armature with said pole faces and pole surfaces in substantial parallelism.

l5. In an alternating current magnet intended 'to be actuated by an alternating current source of substantially constant voltage; an laminated iron yoke having spaced substantially parallel pole faces; a two legged laminated iron armature of U-shaped having pole surfaces at the ends of its legs spaced to align with the pole faces of said yoke; guiding means conning said yoke and armature to a working stroke in relation to one another wherein a pair of substantially equal working gaps is maintained between the yoke and armature, the length of said working stroke being limited to less than about the square root of the area of one of said pole surfaces; and an energizing and flux concentrating winding for each of said working gaps closely surrounding the same, said windings each beiny of a length less than about twice the square root of the associated pole surface area.

16. In an electromagnetic switch to be actuated by an alternating current source of substantially constant voltage a magnetic yoke having a pair of spaced pole faces; a magnetic armature having a pair of spaced pole surfaces each in facing relation to a yoke pole face, for movement into and away from sealing position with the yoke; guiding means confining said armature to a working stroke relative to said yoke wherein a pair of substantially equal working gaps between yoke and armature is maintained and the length of said stroke is limited to less than one and one-half times the square root of the area of one of said pole surfaces; a set of stationary contacts;

a set of movable contacts including prestressed backing springs moved by said armature said movable contacts being located with respect to said armature to make initial contact with said stationary contacts at an intermediate position in the stroke of the armature said backing springs being displaced at full stroke of the armature to transfer the stress thereof to the stationary contacts through the movable contacts; and an energizing and flux concentrating winding for each working gap closely surrounding the same with the turns thereof most remote from the poles located beyond the ends of the working gap when the contacts are in initial contact position but not more than twice the length of the maximum working stroke therefrom, whereby each entire working gap at initial contact engagement is encircled by the associated winding to cause the flux induced by the windings and extending between the poles defining said working gap to be urged inwardly to a substantial degree with respect to the working gap to substantially increase the flux density in said gap with consequent increase in full force.

l7. In an electromagnet adapted to be actuated by an alternating current source of substantially constant voltage a magneti-c yoke having a pair of spaced pole faces; a magnetic armature having a pair of spaced pole surfaces each in facing relation to a yoke pole face, for

astma@ l. 12 l movement into and away from sealing position with the yoke; guiding means confining said armature to a working stro-ke relative to said yoke wherein a pair of substantially equal working gaps between yoke and armature is maintained and the length of said stroke is limited to less than one and one-half times the square root of the area of one of said pole surfaces; and an energizing and ux concentrating winding for each working gap closely surrounding the same with the turns thereof most remote from the poles located not more than twice the length of the working gap therefrom to cause the flux induced by the windings, extending between the poles defining said working gap, to be urged inwardly to a substantial degree with respect to the working gap to substantially increase the flux density in said gap with consequent in crease in pull force.

18. In an electromagnetic actuator the combination comprising a magnetic frame providing a flux path closed on itself divided at two places to form a yoke and a relatively movable armature scalable with said yoke and separable therefrom to present a pair of working gaps each restricted in length to within one and one-half times the square root of the area of an adjacent pole, and a coil for connection to an alternating current source of substantially constant voltage for each of said working gaps each closely surrounding and concentrated about its respective gap.

19. In an electromagnetic actuator the combination comprising a magnetic frame providing a flux path closed on itself divided at two places to form a yoke and a relatively movable armature scalable with said yoke and separable therefrom to present a pair of working gaps, and a coil for connection to an alternating current source of substantially constant voltage for each of said working gaps each surrounding and concentrated about its respective gap with a coil length no greater than twice the square root of a pole face area to confine the ux resulting from excitation thereof to achieve greater flux density in the working gaps than with coils of greater length.

20. In an electromagnetic switch to be actuated by an alternating current source of a substantially constant voltage a laminated magnetic yoke having a pair of spaced pole faces; a laminated magnetic armature biased to move away from said yoke which has a pair of spaced pole surfaces each in facing relation to a yoke pole face t0 present a pair of working gaps; switch actuator means movable with the armature and extending therefrom; guiding means for the armature and actuator means that connes the armature 'to a translatory movement in which substantially equal working gaps are maintained; a set of fixed contacts; a set of movable contacts carried by said actuator means that make initial contact with said fixed contacts at an intermediate position in the stroke of the armature; contact springs compressed upon contact of said movable contacts with said fixed contacts to provide a substantial force for contact engagement and which thereby impose a rapid increase in load upon saidy armature at an intermediate position in the armature stroke; and a pair of operating coils each closely surrounding and concentrated about a working gap to confine and direct the tlux in the gap to increase the flux density and reduce leakage flux thereby developing a rapid rise in pull force as the armature moves to closed position that exceeds the load of said springs and moves the armature through to sealed position with said yoke, the midpoint of the length of each coil being substantially coincident with the mid-point of the length of the associated working gap when the armature is in the stroke position of initial contact engagement.

2l. In an electromagnetic switch the combination comprising a magnetic frame providing a flux path closed upon itself divided at two places to form a yoke and an armature relatively movable in a stroke toward and from said yoke to present a pair of working gaps, means limit ing armature movement to restrict the working gaps to within one and one-half times the square root of the area of an adjacent pole face, a set of stationary contacts, a set of movable contacts carried by said armature making initial engagement with said stationary contacts in an intermediate position in the stroke of the armature, a set of contact bias springs cooperating with said movable contacts interposing a load on said magnetic frame upon engagement of the movable and stationary contacts, and a pair of coils each surrounding a gap and to be connected to a source of alternating current of substantially constant voltage, each coil being concentrated about its respective gap to control and direct a substantial part of the ux resulting from excitation of the coils so as to increase ux density in said working gaps.

References Cited in the le of this patent UNITED STATES PATENTS Wilms et al Sept. 1, 1936 Collier Aug. 8, 1911 Goff Aug. 27, 1929 Bossart Nov. 8, 1932 Stapelton .Tune 12, 1945 Ayers et al J an. 6, 1948 Logan Sept. 6, 1949 Goldberg Mar. 27, 1951 Jarvis et al Feb. 5, 1952 Fisher Oct. 19, 1954 FOREIGN PATENTS Great Britain Feb. 16, 1922 U. s. DEPARTMENT 0E COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,817,729 December 24, 195'7 Leland Eo Lawrence It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Let sers Patent should read as corrected below.

Column 2, line 59, for "dimensions" read dimension u; column 4, line 72, for "to the front" read m: at the front u; column 7, line 6l, after "center of" insert the m; column 8, line 24, for "proportional" read e proportioned n; line 25, for "eonstruetel" read constructed m; lines 53 and 54, after "portionn strike out "of said yoke with a pair of legs extending from opposite ends of the medial portiorU; eolumn ll, line 25, for "an laminated" read a laminated am; line 2'?, for "Umshapedl read Ueshape. line '70, for nfull" read m pull `Signed and sealed this let day of April 1958o (SEAL) Attest:

KARL H AXLINE ROBERT c. wATsoN ttesting Officer Comissioner of Patents U. S. DEPARTMENT OE COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Leland Lawrence It 's hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Let ners Patent should read as corrected below.

Column 2;, line 59, for "dimensione" read dimension n; column w line '72, for "to the front" reed m at June from', ma; column '79 line 61a after "center of" insert uthe w; column 8;, line 243 for "proportional" read e proportioned en; line 25,9 for "oonetruotel" reed me! constructed en; linee 53 and 54 after "portionn strike out "of said yoke with e pair of legs extending from opposite ende of the medial portiorv; column ll, line 25y for "an leunineiedH reed a laminated ee; line 279 for "U`-=ehapedu read Umslielln-:el line '709 for "full" reed pull Signed and Sealed this lst day of April l958 (SEAL) Atest:

KARL H AXLINE noBEnT c. wATsoN Attesting Officer Conmissioner of Patents 

